Previous fuel systems for internal combustion engines have contemplated the vaporization of the liquid fuel in order to improve the mixture thereof with air entering the carburetor and some, such as those shown in U.S. Pat. Nos. 4,000,225, 4,105,004 and 4,106,459 have proposed the use of ultrasonic transducers for this purpose. However, there is no known means for controlling the level of vapor density. Moreover, the exposure of a mixture of air and vapor to heated engine components is highly volatile and hazardous.